Water resistant corrugated paperboard

ABSTRACT

A water resistant corrugated paperboard materail is prepared wherein the paperboard components (medium and liner or liners) are each coated on both sides with a thermoplastic film which acts as a water barrier and a water resistant adhesive for bonding the components together. The corrugated medium is also pre-treated with an internal size treatment to prevent edge wicking, and, for the purpose of corrugating the medium without deleteriously affecting the thermoplastic film, the corrugating rolls are preheated to a temperature slightly less than the melting point of the thermoplastic film while a lubricating material is simultaneously applied to either the corrugating rolls or the medium at the corrugating nip.

United States Patent [191 Hintz et al.

[75] Inventors: Harold L. Hintz; Joseph T. Webb,

both of Charleston, SC.

[73] Assignee: Westvaco Corporation, New York,

[22] Filed: July 21, 1972 21 Appl. No.: 273,835

[52] U.S. Cl 156/208, 117/152, 161/133 [51] Int. Cl B3lf l/22 [58] Fieldof Search 156/208, 210, 133;

[56] References Cited UNITED STATES PATENTS 3,189,502 6/1965 Little156/210 3,308,006 3/1967 Kresse et al. 156/210 3,409,500 1l/1968Strazdins et a1. 162/164 3,445,330 5/1969 Kulick et al. 162/1643,676,247 7/1972 Morris et al. 156/210 FOREIGN PATENTS OR APPLICATIONS879,643 8/1971 Canada Nov; 19, 1974 A OTHER PUBLICATIONS Casey, Pulp andPaper, Second Edition, 1960, pages 1052-1057.

Primary ExaminerGeorge F. Lesmes Assistant ExaminerPaul J. Thibodeau [57] ABSTRACT A water resistant corrugated paperboard materail is preparedwherein the paperboard components (medium and liner or liners) are eachcoated on both sides with a thermoplastic film which acts as a waterbarrier and a water resistant adhesive for bonding the componentstogether. The'corrugated medium is also pretreated with an internal sizetreatment to prevent edge wicking, and, for the purpose of corrugatingthe medium without deleteriously affecting the thermoplastic film, thecorrugating rolls are preheated to a temperature slightly less than themelting point of the thermoplastic film while a lubricating material issimultaneously applied to either the corrugating rolls or the medium atthe corrugating nip.

10 Claims, 3 Drawing Figures 1 WATER RESISTANT CORRUGATED PAPERBOARDSUMMARY OF INVENTION The present invention relates generally to animprovement in corrugated paperboard and specifically to a waterresistant corrugated paperboard packaging material and method forproducing the packaging ma terial.

Corrugated paperboard is widely used in the packaging industry wheresubstantial container strength and an economical construction isdesired. However, the water absorption and moisture penetrationcharacteristics of conventionally manufactured corrugated paper boardhave made it unusable for containers which are subjected to either highhumidity or direct water cooling and/or ice containing conditions. Forinstance, although some attempts have been made to use conventionalcorrugated paperboard as the material of construction for icedcontainers, which use ice as a refrigerant, the water produced by themelting ice soaks into the paperboard so rapidly and produces such aweakening effect on the containers, that complete collapse of thecontainers frequently occurs before the product reaches its destination.Similarly containers which are subjected to hodrocooling conditions,where water is sprayed directly onto the container to maintain thefreshness of the product packaged therein, also suffer deleteriouslyfrom the effects of water penetration and absorption. In addition,shipment and storage conditions frequently subject corrugated paperboardcontainers to high humidity conditions or even exposure to rain or tosnow, over extended periods of time. Under such conditions, even if thecontainer does not weaken to the point of collapse, it is stillineffective to protect the contents of the container from thepenetration of moisture through the walls of the container.

In an effort to overcome the susceptibility of corrugated paperboardcontainers-to structural weakening and failure upon contact with wateror water vapor, it has previously been suggested to coat the paperboardliners or medium, or both, with a water resistant sizing or coatingmaterial. Other suggestions have been directed toward the application ofa thermoplastic film to one or both of the paperboard liners or mediumespecially as taught by Canadian Pat. No. 879,643. Unfortunately, noneof these previously offered solutions serve to produce a completelysatisfactory product. In the case of the water resistant sizing orcoating technique, the problems encountered have included both anunsatisfactory penetration of the water proof material into thepaperboard itself, or, a cracking of the water resistant film when thepaperboard blanks are folded and set-up. With respect to thethermoplastic film laminated products, the high cost of produc@ and thedifficulty in corrugating the thermoplastic coated medium, or inobtaining a good bond between the thermoplastic coated medium and/or theliner or liners on the machine, except at slow speeds, have beenthe'detrimental factors. In addition, no satisfactory means has yet beendeveloped for preventing moisture from wicking into the edges ofthermoplastic coated paperboard material and producing a devastatingweakening effect to the otherwise strong material.

However, by the present invention, each of the limitations anddisadvantages of the prior art methods have been overcome by providingan improved corrugated paperboard and method of manufacturing corrugatedpaperboard to achieve a water resistant corrugated paperboard havingsuperior WVTR (water vapor transmission rate) properties, excellentrigid when wet char acteristics, and good runnability on the machine.Broadly stated, the present invention comprises a method of constructingan improved water resistant corrugated paperboard, which method employsa thermoplastic coating on both sides of the corrugated medium and theliner or liners. Accordingly, the thermoplastic coating serves as boththe water barrier and the water resistant adhesive for bonding thecorrugated paperboard together. In addition, the present invention alsocontemplates the use of a novel size treatment for the corrugated mediumto inhibit any wicking of water from the edges through the coatedmedium. And Finally, the invention further incorporates a combinedheating and lubricating step at the corrugator to insure propercorrugation of the thermoplastic coated medium without cracking or,otherwise deleteriously affecting the thermoplastic coating.

In an illustrative embodiment, the product produced by the presentinvention may be described more specifically as a water resistant rigidwhen wet corrugated paperboard material wherein the paperboardcomponents (medium and liner or liners) are each coated on both sideswith a thermoplastic polyolefin such as polyethlene. The polyolefincoating on both sides of the medium and liner board is preferably in therange of 0.75 1.0 mil thick, or even thicker, and serves as a waterbarrier and as a water-resistant adhesive for bonding the componentstogether. The corrugated paperboard product is produced on conventionalcorrugating equipment, except that in the process of the presentinvention, wicking of water or other moisture from the edges through thelinerboard and medium is controlled by a special synthetic or rosinsizing treatment to both the linerboard and the medium. In addition, forthe purpose of carrying out the process of the invention, i.e., themaking of the novel water resistant corrugated paperboard productdescribed herein, the corrugating rolls on the corrugating machine arepreferably heated to a temperature slightly below the metling point ofthe polyolefin coating while simultaneously a lubricating material isapplied to either the surface of the corrugating rolls, or to both sidesof the medium at the entrance to the corrugating nip. When polyethyleneis used as the preferred polyolefin, the corrugating rolls arepreferably heated to a temperature of between 2l5F., and the preferredlubricating material consists of either a fine plain water spray or awater spray containing a milddetergent.

Thus it maybe stated that the novelty in the presentinvention as regardsthe product produced lies in the use of a thermoplastic polyolefincoating of at least 0.75 mil thick on both sides of both the medium andthe linerboard, in combination with a size treatment to the corrugatingmedium consisting of by weight either at least 0.2% synthetic size, inthe form of a stearic anhydride or succinic anhydride emulsion with 0.30.4% alum, or 0.75 1.0% rosin size with up to 1.5% alum. Sincecorrugating medium is normally unsized, to enable the medium to pick upthe usual starch adhesive when forming conventional corrugatedpaperboard,

the mere fact of applying a size treatment to the corrugating mediumitself, according to the present invention, represents a departure fromthe conventional manufacture of corrugated paperboard. Moreover, thepreferred sizing applied to the medium according to the presentinvention is of a higher level than is normally used even on linerboard.Conventionally, linerboard is internally sized with a rosin/alum systemcontaining up to 0.5% rosin. Accordingly, the sizing treatment of thepresent invention serves to contribute to the surprising resultsobtained with the corrugated paperboard produced, particularly asregards strength retention after a 24 hour water soak test, since thesizing treatment prevents water from wicking through the unprotectededges of the corrugated medium and linerboard, and through holes orcracks that might develop in the polyolefin film coating. In addition,the considerable increase in the waterproofness of the corrugatedpaperboard produced herein may be attributed to the thickness of thepolyolefin film applied which serves to control the overall WVTR of theboard. Moreover, since the crush strength of corrugated paperboard isgreatly reduced by moisture absorption, the thickness of the polyolefinfilm is very significant with respect to the ultimate strength of theboard after soaking.

, With respect to the process for producing the improved water resistantcorrugated paperboard according to the present invention, it wasdiscovered that a sheet of sized, polyolefin coated corrugating medium,with a film thickness in excess of 0.75 mil on each side, could not bereadily corrugated on conventional equipment without the addition of twosignificant steps.

First, it was found that flutes could not be formed in the film coatedmedium without heating the medium to a temperature slightly less thanthe melting temperature of the film. This was accomplished by directlyheating one surface of the medium and indirectly heating the othersurface of the medium by applying heat to one of the corrugating rolls.Secondly, it was found that even with the heated medium, cracks stilltended to develop in the film until a lubricating material was appliedto the medium at the corrugating nip. Thus it is believed that the novelsteps introduced into the art by the present invention, as regardsthe'process described, constitute the limitations whereby the filmcoated corrugating medium is heated to a temperature to render thethermoplastic film soft, and, a fine spray of lubricating water isapplied to the film coated medium or to the corrugating rollsthemselvesat the nip of the corrugating rolls.

For the purpose of bonding the polyolefin coated medium and linerboards,it. was only necessary to apply sufficient heat to one or both of thepolyolefin films to render the surface thereof tacky just prior to thebonding nips. In this manner, the components were heat sealed to oneanother to form the almost completely water resistant paperboardmaterial disclosed herein.

DESCRIPTION OF DRAWING FIG. 1 shows schematically the method ofmanufacturing single face paperboard according to the present invention;

FIG. 2 shows schematically the method of manufacturing double facepaperboard according to the present invention; and,

FIG. 3 is a view in cross-section of the water resistant corrugatedpaperboard produced according to the present invention.

DETAILED DESCRIPTION The present invention is directed to an improvedwater resistant .corrugated paperboard material manufactured in a heatsealing operation from thermoplastic coated paperboard componentswherein the thermoplastic coating is preferably a thermoplasticpolyolefin material having a film thickness on the order of from 0.75 l.0 mil thick. The linerboard component used, in addition to beingprecoated both sides with a polyolefin such as polyethylene, is ofconventional linerboard material. However, the corrugating mediumpreferred is non-conventional in that prior to coating the corrugatingmedium on both sides with a polyolefin film such as polyethylene, thepaperboard material from which the corrugating medium is made isspecially treated with a size treatment comprised of either at least a0.2% synthetic size, in the form of a stearic anhydride or succinicanhydride emulsion in the presence of from 0.3 0.4% alum, or a rosinsize of from 0.75 1.0% rosin in the presence of about 1.5% alum. Inaddition, the thickness of the polyethylene film coating preferred is atleast 0.75 mil or thicker, thus yielding a water resistant corrugatedpaperboard material having excellent rigid when wet characteristics. Thepolyethylene film is preferably extruded simultaneously on both sides ofthe medium (before corrugating) and the linerboard material in a singlepass through an extruder, although two passes coating one side at a timecould be used. The polyethylene film on both sides of the medium and thelinerboard material serves as an excellent water barrier giving goodWVTR properties to the components, and as a water resistant adhesive forbonding the component elements of the corrugated paperboard together.The synthetic or rosin size treatment to the corrugating medium furtherserves to insure a high retention of strength to the corrugatedpaperboard by preventing wicking of water into the medium from the edgesof the paperboard, or moisture penetration through the linerboards intothe medium under severe humidity or water soak conditions. Furthermore,even though the rosin size treatment is considered the preferredtreatment, it is believed that other sizing materials such as asphalticemulsions, alkylketenedimers, waxes and fatty acids could be used withsimilar success.

The invention is also directed to a novel method of manufacturing waterresistant corrugated paperboard wherein the paperboard componentsconsist of one or more linerboard members and at least one corrugatedmedium, each of which is coated on both sides with a polyolefin filmsuch as polyethylene. The method of manufacture for a single faceroperation comprises the preliminary step of applying to both sides of alinerboard component and a sized corrugating medium component athermoplastic film of polyethylene or the like having a thickness inexcess of 0.75 mil. The polyethylene coated corrugating medium is thenconducted to a corrugating section of a conventional corrugating machinewhere the pre-coated medium is preferably slightly pre-heated and passedover a heated corrugator roll and into contact with another heatedcorrugator roll, in the presence of a lubricating water spray shower atthe nip of the corrugator rolls, to form corrugations in the mediumwithout cracking or otherwise deleteriously affecting the polyethylenefilm on the medium. Subsequently, the corrugated medium and the singleface linerboard are adhered to one another by applying to either thelinerboard or corrugated medium, at the bonding surface, a directheating treatment to render the surface of one or both of the componentstacky or slightly melted. At the same time, both the corrugated mediumand the single face linerboard are applied with an indirect heattreatment, through one of the corrugating rolls and through the pressureroll, respectively, to assist in the bonding operation. The heat appliedindirectly to the linerboard and the corrugated medium, through thepressure roll and the corrugating roll respectively, is critical sincetoo much heat causes the coated linerboard or medium to adhere to thepressure roll or corrugating roll and too little heat tends to conductthe required heat for bonding away from the bonding nip.

Similarly, for a double facer operation, the single face board takenfrom the above described operation would be bonded to anotherpolyethylene coated linerboard component in substantially the same way.For example, the single face corrugated board taken from the previousoperation would be applied with a direct heat source on its flutedsurface and the second polyethylene coated linerboard would be directlyheated as before to a temperature slightly less than the melting pointof the polyethylene (less than 200F.) to render the surfaces tacky andbondable. Finally, in the presence of an additional direct heatingsource at the double face combiner station, the single face corrugatedboard and the double face linerboard would be bonded together and thenpassed between a series of cold plates or belts to allow the differentcomponents to be completely bonded into the water resistant corrugatedpaperboard material described herein.

Referring now more particularly to FIG. 1 of the drawing herein, aschematic illustration of the preferred means for manufacturing singleface corrugated paperboard according to the present invention is shown.In FIG. 1, a web of linerboard is shown as being unwound from a roll 11.The linerboard material 10 as previously described is of a conventionaltype, except that for the purposes of the present invention, both sidesthereof have been previously coated with a film of a thermoplasticpolyolefin material such as polyethylene. As stated hereinbefore, thethickness of the film of polyethylene is preferably at least 0.75 mil,but may be between 1.0 5.0 mils in thickness depending upon the type ofduty intended for the final product.-

After being unwound from the roll 11, the coated linerboard web 10 isthen given a pre-heat treatment by the heater 19 prior to beingconducted to the pressure roll 28 as shown. The pre-heat treatment at 19is intended only to condition and instigate a slight softening of theoutside film of polyethylene on the linerboard web 10 prior to enteringthe bonding nip at 18. Accordingly the temperature at the pre-heater 19is only on the order of 200 F. for a typical polyethylene coating.

Meanwhile, still referring to FIG. 1, there is also illustrated a roll13 of corrugating medium 12 which, as previously pointed out, isdecidedly unconventional. For the purposes of the present invention, thecorrugating medium 12, in addition to being pre-coated both sides with afilm of polyethylene or the like, as in the case of the linerboard justdescribed, also includes another special treatment to render it notsusceptible to the edge wicking of moisture in any form. Namely, thecorrugating medium 12 includes a size treatment which maycomprise eitherat least a 0.2% synthetic size in the form of a stearic anhydride orsuccinic anhydride emulsion with from 0.3 0.4% alum, or a rosin size offrom 0.75 1.0% rosin in the presence of about 1.5% alum. The preferredsize treatment comprises a wet end internal treatment, although it isbelieved that a surface size or gate roll size treatment would achievesimilar results providing the latter two treatments were able to obtaingood penetration of the size material into the medium. Accordingly, inview of the fact that conventional corrugating medium is normallyunsized, treatment described is believed to be one of the key factors inthe surprising success of the present invention as will be furtherillustrated later on.

However, for the purpose of carrying out the present invention, the sizetreated and pre-coated medium component 12 is conducted to thecorrugating section 20 of the corrugating machine in a substantiallyconventional manner except that, prior to reaching the corrugating roll23, the outside surface of the polyethylene coated medium 12 is given adirect pre-heat treatment by the pre-heater 17. In the case of apolyethylene coated medium 12, the pre-heater 17 would be set at atemperature of around 200 F. since its purpose is only to slightlysoften the outside film surface of the medium 12 preparatory to enteringthe corrugating nip 20. If other thermosplastic coatings were used onthe medium 12, the te'mperture of the pre-heater 17 would be adjustedaccordingly.

At the corrugating section 20, the medium 12 is passed around a heatedcorrugating roll 23 and then between the roll 23 and another heatedcorrugating roll 22. Each of these rolls 22, 23 are preferablyinternally heated to a temperature slightly less than the melting pointof the polyethylene film, or in the range of from 200f., depending uponthe speed of operation. The temperature applied internally through thecorrugating rolls 22, 23 is critical as pointed out hereinbefore sinceit must be high enough to permit the coated medium 12 to be corrugatedyet low enough to prevent the coating from sticking to the corrugatingrolls. Thus it may be seen that both surfaces of the pre-coated mediumare heated, by the pre-heater 17 and the corrugating roll 23, prior toentering the corrugating nip 20 to render the medium 12 pliable andsusceptible to being corrugated without cracking or otherwisedeleteriously.

affecting the thermoplastic coating.

However, as pointed out hereinbefore, the fact that the medium is heatedbefore corrugating is not a sufficient preparation for successfullycorrugating a precoated medium having either a high coat weight (i.e., athick coating in excess of 0.75 1.0 mil), or at a high speed.Accordingly, a pair of lubricating sprays 24, 25 are added to thecorrugating section on 20 to either spray both sides of the medium 12before entering 79 the corrugating nip 20, or to spray directly onto thecorrugating rolls 22, 23.

FIG. 1 illustrates a spray nozzle24 adjusted to lubricate either theinside surface of the medium 12 or directly onto the roll 23. Similarly,a spray nozzle 25 is also shown in FIG. 1 which lubricates the outsidesurface of the medium 12 or the roll 22. In this manner, the heatedcorrugating rolls 22, 23 allow the medium to be bent in forming theflutes and the lubricant, in the form of either a fine mist of plain ordetergent containing water, acts as a means for reducing friction as thecoated medium 12 is forced between the corrugating rolls 22, 23. Itshould further be appreciated that only a schematic view of theinvention is illustrated, and for a full scale model a shower pipe or aplurality of nozzles would be needed throughout the width of thecorrugating section 20.

After passing between the nip of the corrugating rolls 22, 23, the nowcorrugated medium 12 is shown as being bonded to the single facelinerboard at the combining station 18. To accomplish this step, thebonding surface of the linerboard 10, as previously stated, is preheatedat 19 to a temperature of about 200 F., and brought into nip engagementwith the indirectly heated corrugated medium 12 between the internallyheated (l50200 F.) pressure roll 28 and the corrugating roll 22. Again,as pointed out hereinbefore, the temperature of the pressure roll 28must not be too high or the coated linerboard will tend to adhere to theroll. Moreover, the temperature of the pressure roll 28 cannot be toolow or the heat necessary for bonding will be conducted away from thebonding nip. To assist in the bond and insure that the proper surface ofthe corrugated medium becomes tacky, additional direct heat is appliedto the bonding surface of the medium 12 and/or the linerboard 10 by theheating means 29. For the purposes of the present invention, it has beenfound that a hot air burner having a capacity of at least 5000,000Btu/hr. connected to a large blower, is sufficient to produce a hot airstream on the order of 450F.

and thus insure a good bond between the medium and the linerboard atrespectable machine speeds (in excess of I50 ft./min.) After thecombiner station, the now single face corrugated paperboard is passedaround a first guide roller 30 and over a second guide roller 31 andonto an endless conveyor 32 upon which the single face board isaccumulated for the purpose of cooling the board and allowing the bondto become completely set.

FIG. 2 illustrates a preferred method for making double face corrugatedpaperboard from the single face board obtained from FIG. 1. In thisview, the single face board accumulated on the endless conveyor 32 isconveyed over a guide roller 33 past a preheat station 34 and into a nipcreated between the cold plates or belts at 37, 38. Meanwhile, thelinerboard 14 is stored on a roll and as before, with the linerboard 10,has coated on both sides thereof a film of a thermoplastic material suchas polyethylene. The linerboard 14 is accordingly passed around a guideroller 39 and past a pre-heat station 36 and into the nip at 37, 38. Ateach pre-heat station 34, 36, the bonding surfaces of the fluted medium12 and the linerboard 14 are each preheated to about 200F wherepolyethylene film is used, and for the purpose of rendering the surfacestacky or melted enough to become bonded, a second heating means 35 isarranged to provide direct heating treatments in excess of 450F to thefluted medium and/or liner thereby a good bond. Finally, the double facecorrugated paperboard is then permitted to cool between the coolingplates or belts 37, 38 until the bond sets.

In order to demonstrate the surprising results obtained as regards thewet strength retention of the corrugated paperboard produced inaccordance with the present invention, tests were conducted whereinboxes were constructed from different kinds of corrugated paperboard andsubjected to top-to-bottom compression strength and 24 hours water soakstrength retention measurements. For the purpose of comparison, the

boxes were constructed from a conventional, nontreated corrugatedpaperboard referred to as Domestic, a cascade wax treated corrugatedpaperboard wherein the box blanks were coated with wax on each side in acurtain coater, and referred to as Cascade Waxed; a wax-impregnatedcurtain coated corrugated paperboard product called Cote-A-Cor 600; and,the polyethylene coated product of the present invention referred to asPolyboard. The Polyboard sample had a 0.75- 1.0 mil thick film ofpolyethylene applied to each side of each component. In each case, thebasis weight of the different paperboard components and the corrugatedpaperboard constructions were substantially the same except that for theCote-A-Cor 600 box and the Cascade Waxed box, the fluted medium wasC-flute and for the other examples, the fluted medium was A-flute. Forthe purposeof explanation, the number of flutes-per linear foot isgreatest with C-flute, and the height of the flutes between the twofacings is greatest with A-flute. Therefore, it might be expected thatC-flute corrugated paperboard would be somewhat stronger than A-flutecorrugated paperboard. Some of the sample boxes were tested for theirtop-to-bottom compression strength under standard atmospheric conditions, and other samples were subjected to a 24 hour water soakcondition and then tested for top-to-bottom compression. Based on thetest decribed, the following data was obtained.

TABLE I Top-to-Bottom Compression Strength and Strength RetentionPaperboard Std. Cond. 24 Hour Soak Strength Grade (lbs) (lbs) Ret. 7r-

Polyboard 596 483 81.0 Cascade Waxed 7l3 91 12.8 Cote-A-Cor 600 872 829.4 Domestic 578 Delaminated For the samples tested, the load the boxeswere able to withstand under standard conditions varied depending uponthe kind of fluted medium used as explained before. However, the amountof strength retained by each box after the 24 hour water soak treatmentwas directly related to the water absorption characteristics of thepaperboard material used. In the case of the box constructed fromDomestic paperboards, water absorption was immediately observed when thebox was submerged in the water tank and delamination occurred. For thebox constructed from Cote-A-Cor 600 and the Cascade Waxed box, the waterabsorption was not immediate upon dunking, but the wicking of the flutedmedium plus the moisture absorbed through the linerboard to the flutedmedium in each case was severe enough to permit internal collapse ofthepaperboard material when load was applied after the 24 hour soak. In thecase of the Polyboard boxes, the water absorption was very low, andsignificantly, there was practically no visible wicking of the sizetreated corrugated medium compared with the other boxes, the loss instrength of the Polyboard box after soaking was only slight, as noted inthe data, and the loss that did occur was directly related to finecracks or pin holes in the polyethylene coating applied to thecomponents.

In order to further demonstrate the dramatic retention of wet strengthachieved by applying a size treatment to the conventionally unsizedcorrugated medium a second test was conducted wherein corrugatedpaperboard samples were prepared using various types of medium. Asbefore, conventionally sized linerboard was used, coated on both sideswith a 0.75 1.0 mil and improved dry strength of the thermoplasticcoated paperboard described herein, it is conceivable that thepaperboard components used could be of a lower basis weight materialthan that normally required for a given thick fil o p y y and thislinerboard Was purpose. Thus, cost savings in raw material could bebonded to different examples of corrugated medium achieved with thenovel corrugated paperboard matealso coated on both sides with a 0.751.0 mil thick .rial des ibed h i film of polyethylene. The differentcorrugated medi- It may be seen that the water resistant corrugatedpaums chosen were a typical unSiZed medi a medium perboard describedherein is capable of obtaining and sized with rosin 1n the preferrmanner Set forth retaining excellent rigid when wet strengthcharacterishefem 10% r051n Wlth 1.5% alum), 3 medium tics heretoforeunattainable by any competitive prodsized with a We eng resin (ureaformaldehyde), uct. In fact, the corrugated paperboard disclosed, withand amedium fabricated from conventional linerboard a i d di as f d,hibi h b t i id material. The results of the test are shown below and hwet -form n e'of any corrugated paperboard illustrate the surprising wetstrength retention that ever tested.

could be expected by sizing the corrugating medium in accordance withthe present invention.

TABLE II Thus, from the foregoing discussion, it should be apparent thata substantial improvement in water resis- Topao-Bottom CompressionStrength and Strength Retention (sized med.)

Thus it may be seen from the results set forth above that when a regular(unsized) corrugated medium was used, the compression strength aftersoaking for 24 hours was only 2.37% of its orginal value. The additionof a wet strength resin to the medium raised the strength retention onlyto and, even with conventionally sized linerboard as the medium, the wetstrength retention amounted only to 31.6%. Since linerboard isconventionally sized, it could have been expected that with thelinerboard as the medium, the strength retention would have been higher.Later observations found that the Polyboard constructed using linerboardas the corrugated medium were not properly coated in the polyethyleneextruder. Hence, this problem was believed to be a contributing factorin the low strength retention found with the linerboard corrugatedmedium. However, the only dramatic increase in wet strength retentionwas obtained when the corrugating medium was treated according to thepresent invention.

Other experiments illustrated that polyethylene was not the onlymaterial that could be used to coat the paperboard components. Whenlinerboard and corrugat; ing medium having a hot melt adhesive coatingwere tested, the moisture absorption and strength retention of theseelements were found to be similar to the preferred Polyboard productdescribed herein.

In addition, the use of a hot melt adhesive to coat the paperboardcomponents would offer an excellent tant corrugated paperboard has beenachieved by the present invention. Accordingly, even though only aschematic showing of the details of the present inven- 5 tion, it shouldbe apparent that certain modifications and innovations could be added tothe disclosure without departing from the principles of the inventionset forth. Therefore, what is intended to be covered by 0 Letters Patentis embraced in the following claims.

We claim:

1. In the process of manufacturing water resistant corrugatedpaperboard, consisting of at least one linerboard component having bothsides thereof coated with a film of thermoplastic material, and at leastone corrugating medium, component having both sides thereof coated witha film of thermoplastic material, the steps of: v

a. pretreating the said corrugating medium component with an intervalsize treatment to selected from the group consisting of 0.75-1.0% rosinwith about 1.5% alum, 0.2% stearic anhydride with O.30.4% alum, or 0.2%succinic anhydride with 0.30.4% alum inhibit the water wickingtendencies thereof;

b. applying a direct pre-heat treatment to one side of the said coatedand sized corrugating medium prior to entering the corrugating nip of acorrugating machine; I

c. applying an indirect pre-heat treatment to the opposite side of thesaid coated and sized corrugating medium by heating at least one of thecorrugating rolls of the corrugating machine;

d. applying a lubricating treatment in the form of a water spray or awater spray containing detergent to the corrugating nip of thecorrugating machine;

tion have been presented for the purposes of descrip-' e. applying adirect pre-heat treatment to one side of the said coated linerboardcomponent prior to entering the bonding nip of a corrugating machine;

f. applying an indirect pre-heat treatment to the opposite side of thesaid coated linerboard component by heating the pressure roll of thecorrugating machine;

g. applying an additional direct heating treatment at the bonding nip ofthe corrugating machine to render the thermoplastic film surface on oneof the components tacky and bondable;

h. pressing the tacky and bondable surface of one of the components intocontact with the other of said components at the bonding nip; and,

i. cooling the laminate thus formed to allow the bond to set.

2. The process of claim 1 wherein the thermoplastic material is athermoplastic polyolefin.

3. The process of claim 2 wherein the thermoplastic polyolefin ispolyethylene.

4.-The process of claim 3 wherein the polyethylene film thickness is inthe range of from 0.75 1.0 mil thick.

5. The process of claim 4 wherein the direct pre-heat treatments forboth the corrugated medium components and the linerboard component arein the range of 200F.

6. The process of claim 5 wherein the temperature of at least one of thecorrugating rolls is in the range of from l502l5F.

7. The process of claim 6 wherein the lubricating treatment is applieddirectly to both sides of the corrugated medium.

8. The process of claim 7 wherein the lubricating treatment is alsoapplied to the corrugating rolls of the corrugating machine.

9. The process of claim 10 wherein the additional direct heatingtreatment is of a temperature in excess of 450F.

10. The process of claim 9 wherein subsequent components of the laminateare bonded to the laminate formed by pre-heating the bonding surfaces ofat least two of the components to a temperature in the range of about200F. and applying additional direct heating treatments in excess of450F. at the bonding nip to render the polyethylene film surface on oneof the components tacky and bondable.

1. IN THE PROCESS OF MANUFACTURING WATER RESISTANT CORRUGATEDPAPERBOARD, CONSISTING OF AT LEAST ONE LINERBOARD COMPONENT HAVING BOTHSIDES THEREOF COATED WITH A FILM OF THERMOPLASTIC MATERIAL, AND AT LEASTONE CORRUGATING MEDIUM COMPONENT HAVING BOTH SIDES THEREOF COATED WITH AFILM OF THERMOPLASTIC MATERIAL, THE STEPS OF: A. PRETREATING THE SAIDCORRUGATING MEDIUM COOPONENT WITH AN INTERVAL SIZE TREATMENT TO SELECTEDFROM THE GROUP CONSISTING OF 0.75-1.0% ROSIN WITH ABOUT 1.5% ALUM, 0.2%STEARIC ANHYDRIDE WITH 0.3-0.4% ALUM OR 0.2% SUCCINIC ANHYDRIDE WITH0.3-0.4% ALUM TO INHIBIT THE WATER WICKING TENDENCIES THEREOF; B.APPLYING A DIRECT PRE-HEAT TREATMENT TO ONE SIDE OF THE SAID COATED ANDSIZED CORRUGATING MEDIUM PRIOR TO ENTERING THE CORRUGATING NIP OF ACORRUGATION MACHINE; C. APPLYING AN INDIRECT PRE-HEAT TREATMENT TO THEOPPOSITE SIDE OF THE SAID COATED AND SIZED CORRUGATING ROLLS OF THE CORRHEATING AT LEAST ONE OF THE CORRUGATING ROLLS OF THE CORRUGATINGMACHINE; D. APPLYING A LUBRICATING TREATMENT IN THE FORM OF A WATERSPRAY OF A WATER SPRAY CONTAINING DETERGENT TO THE CORRUGATING NIP OFTHE CORRUGTING MACHINE.
 2. The process of claim 1 wherein thethermoplastic material is a thermoplastic polyolefin.
 3. The process ofclaim 2 wherein the thermoplastic polyolefin is polyethylene.
 4. Theprocess of claim 3 wherein the polyethylene film thickness is in therange of from 0.75 - 1.0 mil thick.
 5. The process of claim 4 whereinthe direct pre-heat treatments for both the corrugated medium componentsand the linerboard component are in the range of 200*F.
 6. The processof claim 5 wherein the temperature of at least one of the corrugatingrolls is in the range of from 150*-215*F.
 7. The process of claim 6wherein the lubricating treatment is applied directly to both sides ofthe corrugated medium.
 8. The process of claim 7 wherein the lubricatingtreatment is also applied to the corrugating rolls of the corrugatingmachine.
 9. The process of claim 10 wherein the additional directheating treatment is of a temperature in excess of 450*F.
 10. Theprocess of claim 9 wherein subsequent components of the laminate arebonded to the laminate formed by pre-heating the bonding surfaces of atleast two of the components to a temperature in the range of about200*F. and applying additional direct heating treatments in excess of450*F. at the bonding nip to render the polyethylene film surface on oneof the components tacky and bondable.